In sewing machines equipped with underbed thread trimming devices for severing the needle thread, it is known to correlate the release and reestablishment of the needle thread tension and the preparatory and active strokes of a needle thread wiper with the operation of the thread trimmer to facilitate a predictable and efficient operation.
It is preferable, for instance, for the thread wiper to partake of its preparatory or thread engaging movement while the thread is taut before it is severed, and for the active thread wiping stroke to occur after the thread is severed to prevent the thread from catching in the work which might cause a long thread end to be pulled out as the work is removed. Similarly, it is desirable for the needle thread tension to be released prior to thread severing to facilitate the drawing down of sufficient thread to preserve appropriate thread end length for starting the next line of stitching, and for the needle thread tension to be restored during the thread wiper operation thus forcing the wiper to withdraw the severed thread end from the work rather than drawing additional thread from the supply.
A construction is known in which one solenoid is connected directly to both the thread wiper and to the tension release mechanism. In this known construction in response to electrical signals related to particular stages of operation of the thread trimmer, the wiper is retracted and the tension is released directly as a result of the operation of the solenoid whereas the wiper is given an active stroke and the tension is restored when the solenoid is deenergized. In the known construction a large and powerful solenoid is required to provide the driving forces to move the wiper and to release the tension.